This invention relates generally to a neutral point clamped (NPC) or diode clamped multilevel converter, and, more specifically, to a method for protection of a multilevel converter.
A diode clamped multilevel converter is generally used in high power industrial applications such as variable speed drive (VSD) systems or in energy conversion applications such as a solar (or photovoltaic) or wind power generating systems. Multilevel converters synthesize a sinusoidal voltage by several levels of voltages, typically obtained from capacitor voltage sources. A three level converter includes two capacitor voltages in series. Each phase leg of the three-level converter has four switching devices in series and two diodes to clamp to the capacitor center point.
The switching devices in a multilevel converter experience high electrical and thermal stress during short-circuit conditions. For example, if there is large power loss within the switching device due to electrical stress or overcurrent conditions, the switching device overheats and may experience thermal breakdown. Once a switching device is broken down or destroyed, without protective measures, a chain reaction may occur and result in destruction of the other switching devices within the converter bridge. Some techniques for protecting the converter bridge include employing a snubber circuit or a crowbar circuit. However, use of additional hardware to is a costly and complex solution.
Therefore, it is desirable to provide a method and a system that will address the foregoing issues.